Milling cutters, for example, ball end milling cutters, very often have to withstand high axial cutting forces whilst remaining securely fastened to the tool holder. It is important in such cases that the retaining screw be protected from the cutting forces; otherwise it may plastically bend or even break.
U.S. Pat. No. 5,951,213 discloses a milling cutter which has a shaft and a rounded end head that has two plate seats at the head, which are diametrically opposite each other. The plate seats are adapted to receive asymmetrical cutting inserts each having a central through hole. The cutting inserts are identical, and are plate-like elements of generally pointed, elongated leaf or prune-pit shape, defining two end points or tips, and having, each, two dissimilar cutting edges. When one cutting edge is worn, an insert can be indexed (rotated) about its through hole and repositioned in a different plate seat to present the other cutting edge for cutting operations.
However, there are some disadvantages. Firstly, the maximum number of inserts that can be attached to the cutting tool is limited to two. Secondly, the geometry of the insert is such that the contour of the cutting edge of the insert consists of a quarter-circle (that is, where the angle subtended at the centre of the circle is 90°) and an extending straight line. This limits the maneuverability of the cutting tool. Thirdly, when the cutting tool is drilling in a substantially downward or upward motion, none of the abutment surfaces are perpendicular to the direction of the drilling, thus avoiding optimal absorption of the axial forces.
U.S. Pat. No. 3,466,720 discloses a reversible throw-away cutting insert for mounting on an appropriate tool holder, having symmetrically disposed equal upper and lower faces forwardly diverging from each other.
However, the V-shaped notches are used for insert locating and ease of mounting and do not serve the purpose of absorbing any axial forces. Secondly, the cutting edge is suitable for machine lathe turning of a cylindrical work piece, and is not appropriate for any type of drilling.
U.S. Pat. No. 6,270,292 discloses a cutting insert for mounting on a tool holder of a cutting tool, comprising a front cutting portion having an operative front surface associated with at least one cutting edge, and a trailing mounting portion. The trailing mounting portion has upper, lower and side walls extending from the front cutting portion to a rear abutment wall of the trailing mounting portion oriented generally co-directionally with said operative front surface.
While the geometry of the insert has been optimized so as to enable it to have relatively long cutting edges with trailing mounting portions being of substantially reduced dimensions, the cutting tool can not be used for making ball-end shaped holes. In addition, at least six cutting inserts must be installed in the tool holder, which is time consuming when inserts need to be replaced.
US Published Patent Application 2010/0124465 discloses a cutting insert that is capable of being mounted within an insert-receiving pocket of a ball end mill. The cutting insert includes a first substantially planar surface, a second substantially planar surface, a first curvilinear side surface, a second curvilinear side surface, a first pair of cutting edges formed at an intersection between the first substantially planar surface and the first and second curvilinear side surfaces, and a second pair of cutting edges formed at an intersection between the second substantially planar surface and the first and second curvilinear side surfaces. The first substantially planar surface is substantially parallel to the second substantially planar surface. The cutting insert is mirror symmetric about all three axes. Thus, for example, when one member of a given pair of cutting edges is worn, the insert can be indexed (rotated) about its central through hole, i.e., about its z-axis to present the other member of that pair for cutting operations.
Again, there are some notable disadvantages. Firstly, the maximum number of inserts that can be attached to the cutting tool is limited to two. Secondly, the geometry of the insert is such that the contour of the cutting edge of the insert consists of a quarter-circle (that is, where the angle subtended at the centre of the circle is 90°). This limits the maneuverability of the cutting tool. Thirdly, when the cutting tool is drilling in a substantially downward or upward motion, none of the abutment surfaces are perpendicular to the direction of the drilling, thus avoiding optimal absorption of the axial forces.